1. Field of the Invention
The present invention relates to a fuel cell stack formed by stacking a membrane electrode assembly and separators in a stacking direction. The membrane electrode assembly includes a pair of electrodes and an electrolyte membrane interposed between the electrodes. The fuel cell stack includes rectangular end plates provided at opposite ends in the stacking direction. Passages that function at least as fluid supply passages or fluid discharge passages for a coolant or a reactant gas are formed along the end plates in a longitudinal direction, and extend through the fuel cell stack.
2. Description of the Related Art
A solid polymer electrolyte fuel cell, for example, employs a membrane electrode assembly, which includes an anode, a cathode, and an electrolyte membrane interposed between the anode and the cathode. The electrolyte membrane comprises a polymer ion exchange membrane. The membrane electrode assembly is sandwiched between separators to form a power generating cell. During use, a predetermined number of such power generating cells are stacked together, and end plates are provided at opposite ends in the stacking direction to thereby form a fuel cell stack.
In the fuel cell, a fuel gas flow field for supplying a fuel gas to the anode and an oxygen-containing gas flow field for supplying an oxygen-containing gas to the cathode are formed in surfaces of the separators. Further, a coolant flow field for a coolant is formed along surfaces of the separators.
In the end plate, which is provided on at least one end in the stacking direction, there are provided a fuel gas supply passage for supplying the fuel gas to the fuel gas flow field, a fuel gas discharge passage for discharging the fuel gas, after consumption thereof, from the fuel gas flow field, an oxygen-containing gas supply passage for supplying an oxygen-containing gas to the oxygen-containing gas flow field, an oxygen-containing gas discharge passage for discharging the oxygen-containing gas, after consumption thereof, from the oxygen-containing gas flow field, a coolant supply passage for supplying the coolant to the coolant flow field, and a coolant discharge passage for discharging the coolant used for cooling from the coolant flow field.
For example, in the fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2000-164238, as shown in FIG. 8, a fuel gas supply through hole 2a, a coolant water supply through hole 3a, and an oxygen-containing gas supply through hole 4a are provided at an end of one of the pressure plates (end plates) 1, whereas a fuel gas discharge through hole 2b, a coolant water discharge through hole 3b, and an oxygen-containing gas discharge through hole 4b are provided at the other end of the pressure plate 1.
An inlet hole assembly pipe 5a and an outlet hole assembly pipe 5b are attached to the pressure plate 1. The inlet hole assembly pipe 5a includes a fuel gas inlet hole pipe 6a fitted to the fuel gas supply through hole 2a, a coolant water inlet hole pipe 7a fitted to the coolant water supply through hole 3a, and an oxygen-containing gas inlet hole pipe 8a fitted to the oxygen-containing gas supply through hole 4a. The fuel gas inlet hole pipe 6a, the coolant water inlet hole pipe 7a, and the oxygen-containing gas inlet hole pipe 8a are connected together by a frame 9a. 
The outlet hole assembly pipe 5b includes a fuel gas outlet hole pipe 6b fitted to the fuel gas discharge through hole 2b, a coolant water outlet hole pipe 7b fitted to the coolant water discharge through hole 3b, and an oxygen-containing gas outlet hole pipe 8b fitted to the oxygen-containing gas discharge through hole 4b. The fuel gas outlet hole pipe 6b, the coolant water outlet hole pipe 7b, and the oxygen-containing gas outlet hole pipe 8b are connected together by a frame 9b. 
In Japanese Laid-Open Patent Publication No. 2000-164238, due to the internal pressure of the fuel cell stack, bending stresses are applied to the pressure plate 1, and the pressure plate 1 may become deformed undesirably. In particular, in the event that the pressure plate 1 has a rectangular shape, a relatively large deformation may occur in the longitudinal direction thereof.
In such a structure, where the inlet hole assembly pipe 5a and the outlet hole assembly pipe 5b are made of resin and are provided along the pressure plate 1 in the longitudinal direction, at ends in the longitudinal direction of the inlet hole assembly pipe 5a and the outlet hole assembly pipe 5b, forces in opposition to the bending force tend to be generated easily, due to the rigidity of the resin. Thus, the ends of the inlet hole assembly pipe 5a and the outlet hole assembly pipe 5b may peel off from the pressure plate 1, and cracks or the like may occur at ends of the inlet hole assembly pipe 5a and the outlet hole assembly pipe 5b. 